I had exactly this issue on my yacht, although I did change my hot water tank arrangement in the process of fixing it.
Previously I had a pressurised hot water tank which was supplied with pressure from the cold water inlet. the water pressure came from my Jabsco
pressure pump. Due to distances up and down the length of the boat
(and the various routes required to get from pump to tank to taps etc) this meant that the cold water travelled maybe 5m-fore from the pressure pump to the shower taps; but the hot water tank was perhaps 5m-aft of the pump. So overall, to get pressure to the hot tap in the shower the pump had to go along around 15m of pipe. This meant that the temp was always yoyo-ing.
Then to get a bigger tank I changed my setup, still with the standard Jabsco
pressure pump which supplied pressure to the cold taps but now the much bigger hot water tank (which is a vented tank) has a header just above. Then on the hot water outtake of the hot water tank I fitted a mixer valve set to get the water at a good temp for showers and washing
up. I then fitted another jabsco pressure pump to take the warm water from the mixer valve adn pressurise it into the hot water pipes.
The result is that the new pump alone pushes a ready supply of water upto the shower, with great results :-)
The Jabsco support ppl advised that the pumps can handle upto 55 Deg C happily. As I have set the mixer way below this, I don't expect the 'warm' pump to have any issues. around 8 months so far without a problem.